At present, organic resin film laminated metal sheets are widely used in various fields such as electrical components, furniture and building materials. In general, there are two well known methods for continuous lamination of an organic resin film on the surface of the metal sheet. The first method is one in which an adhesive coated metal sheet is used. Namely, at first the adhesive in which thermosetting resin is mainly dissolved in a solvent, is coated on the surface of the metal sheet. After curing the adhesive, the organic resin film is laminated on the surface of the metal sheet and then is heated in an oven with a large heat capacity for several minutes. After that, the organic resin film laminated metal sheet is cooled.
The second method is one in which an adhesive coated organic resin film is used. In this case, the organic resin film is laminated on the surface of the metal sheet and then is also heated in the same type of oven as in the first method.
However, the organic resin film laminated metal sheet can not be produced at high speed by these methods because a long time is required for curing the adhesive and heating after lamination of the organic resin film. In some cases, the characteristics of the organic resin film are deteriorated by heating for a long time and the laminated organic resin film may be peeled off from the surface of the metal sheet by severe forming because the formability of the adhesive is not good.
Recently, a process for laminating a biaxially oriented polyester resin film to a metal sheet without an adhesive such as thermosetting resin has been proposed in U.S. patent application Ser. No. 398,694, filed July 15, 1982. This method comprises laminating a biaxially oriented polyester resin film on the metal sheet which is heated to a temperature above the melting point of said polyester resin film and then immediately quenching. This method is suitable for the continuous lamination of the polyester resin film to the metal sheet at high speed, because sufficient bonding strength of the polyester resin film to the metal sheet is obtained by heating for a short time. However, in some applications, the polyester resin film laminated metal sheet by this method is reheated at a temperature of 160.degree. to 200.degree. C. for about 10 to 30 minutes, because color printing or lacquer coating on the other side of the metal sheet wherein the polyester resin film is laminated is carried out before forming. In this case, a part of polyester resin film may be peeled off in a severely formed part after a treatment by hot water or hot steam, because an amorphous polyester resin layer formed at the interface of the biaxially oriented polyester resin film and the metal sheet by heating at a temperature above the melting point of polyester resin film is recrystallized by reheating at a temperature of 160.degree. to 200.degree. C. for a long time. Therefore the polyester resin film laminated metal sheet produced by this method is not sufficiently used for the applications wherein lacquer coating or color printing is required.
Furthermore, in Japanese Patent Publication No. Sho 57-22750, a copolymerized polyester resin film is used for the lamination to the metal sheet without an adhesive such as a thermosetting resin. However, this resin film is only used for this limited application because this film is expensive compared with epoxy phenolic lacquer which is widely used in the can producing industry.
On the other hand, in the can producing industry, a continuous lacquer coating at high speed on the metal sheet has been investigated in order to decrease the production cost. However, such a process is too difficult to practice industrially, because a lacquer which can be cured by heating for a few seconds has not yet been developed. Moreover, the corrosion resistance after severe forming of the lacquer coated metal sheet is poor as compared with that of the organic resin film laminated metal sheet.
Accordingly, it is the first objective of the present invention to provide a metal sheet covered with biaxially oriented polyester resin film having an excellent adhesion of polyester resin film to the metal sheet and excellent corrosion resistance in a severely formed part even after a treatment by hot water or hot steam, more specifically, in the formed part after reheating for curing the color printing ink or lacquer at the temperature of 160.degree. to 200.degree. C. for a long time, as compared with that of the lacquer coated metal sheet and other organic resin film laminated metal sheets.
It is the second objective of the present invention to provide a method for the continuous lamination of biaxially oriented polyester resin film on one or both surfaces of the metal sheet at high speed.